Bulk material analyser for on-conveyor belt analysis

ABSTRACT

The present invention related generally to bulk material analyzers suitable for the direct on-line analysis of materials such as coal and minerals. It is targeted particularly at direct on-conveyor belt analysis. It includes: a shielded enclosure defining an analysis zone within it and having a passageway through it to allow transport of bulk material through the analysis zone. At least one neutron source ( 1,4 ) and at least two gamma ray detectors ( 3,5 ) are disposed within the enclosure to measure gamma-rays produced in the bulk material by both the neutron inelastic scatter and thermal neutron capture processes. A neutron source and a gamma-ray detector are arranged in either a transmission or backscatter geometry. A second gamma-ray detector is arranged either in a transmission or backscatter geometry with or without using a second neutron source. The arrangement of detectors is such that the bias in the spatial response of each, that is caused by the relative attenuation of neutrons and gamma-rays in the bulk material, at least partly compensates for bias in the spatial response of the other.

TECHNICAL FIELD

The present invention relates generally to bulk material analyserssuitable for the direct on-line analysis of materials such as coal andminerals. It is targeted particularly at direct on-conveyor beltanalysis.

BACKGROUND ART

A key requirement for direct on-conveyor belt analysis is the ability tomeasure parameters of interest, such as elemental composition,independently of both horizontal and vertical segregation andindependently of changes in belt loading.

Both neutron inelastic scatter (NIS) and thermal neutron capture (TNC)gamma-ray techniques have the advantages of using highly penetratingradiation so that measurements are averaged over a large volume ofmaterial on a conveyor belt. They are also capable of the simultaneousquantitative determination of many elements.

Thermal Neutron Capture (TNC) Gamma-Rays

The most widely used technique for on-line bulk material analysis isthat based on thermal neutron capture (TNC) gamma-rays (sometimesreferred to as the prompt gamma neutron activation analysis (PGNAA)technique). The TNC technique involves bombarding a bulk sample withneutrons from a radioisotope source, usually 252Cf. The 252Cf neutrons(average energy 2.3 MeV) are slowed down to thermal energies (about0.025 eV) by collisions either in the sample or in an external moderatorand then captured by the nuclei of elements present in the sample. Thecapture process in most cases is accompanied by the immediate release ofenergetic gamma-rays which are characteristic of the element. In mostmaterials the capture gamma-rays form a complex spectrum of energies,which is capable of interpretation to provide analytical information onthe proportion of the various elements present in the sample.

In previous applications of the TNC technique to on-conveyor beltanalysis, spatial uniformity is controlled by the use of multiplesources and detectors in transmission geometry together with neutronmoderators external to the sample. These external moderators are used tocontrol the thermal neutron flux distribution in the sample to produce amore uniform spatial sensitivity.

In another development, an on-belt analyser has been described whichcomprises a 14 MeV pulsed neutron generator and a gamma-ray detectorlocated on opposite sides of a conveyor belt. The neutrons are sloweddown to thermal energies using heavy metal and polyethylene shields andTNC gamma-ray spectra are measured. However, the problem of spatialsensitivity is not addressed as it is assumed that the material ishomogeneous and of constant profile on the belt.

Neutron Inelastic Scatter (NIS) Gamma-Rays

In the NIS technique, fast neutrons undergo direct inelastic scatterreactions with the nuclei of elements in a sample resulting in theproduction of prompt gamma-rays which are characteristic of the elementspresent. For NIS to occur the energy of the incident neutrons must begreater than the energy of the gamma-rays. Suitable high-energy neutronsources are 241 Am-Be (average neutron energy about 4.5 MeV), fastneutron generators (neutron energy 2.45 or 14 MeV) and 252Cf (averageneutron energy 2.35 MeV). Generally higher energy neutron sources arebetter suited to NIS applications.

NIS and TNC are in many ways complementary since an element that may notbe sensitive to NIS may be highly sensitive to TNC and vice versa. Forexample, carbon is easily determined using NIS but is only weaklyexcited in TNC; on the other hand, hydrogen is readily determined usingTNC but it produces no NIS gamma-rays. By using a high-energy neutronsource NIS is readily combined with TNC techniques to determine theconcentration of most of the major elements in a wide range of samples.An example of a successful application combining NIS and TNC is thedetermination of carbon, hydrogen, ash and chlorine in low rank coal ina by-line geometry.

The NIS technique has been developed for a number of industrialapplications on relatively homogeneous materials in sample by-lines.However applications to on conveyor belt analysis have not beenattempted partly because of problems with spatial sensitivity of thetechnique. In a backscatter geometry the effective depth of penetrationinto the sample is limited. For the example of 4.43 MeV NIS carbongamma-rays from coal using a 238 Pu—Be source, 50% of the measuredgamma-rays originate in the first 50 mm of coal and 80% originate in thefirst 100 mm.

SUMMARY OF THE INVENTION

The invention is a bulk material analyser including: A shieldedenclosure defining an analysis zone within it and having a passagewaythrough it to allow transport of bulk material through the analysiszone. The shielded enclosure may be made of a material that contains ahigh hydrogen density, often combined with a material of high neutroncapture cross section such as a compound of boron or lithium. Thepurpose of the shielded enclosure is to provide radiation shielding forpersonnel. In use, the bulk material is transported though the analysiszone on a conveyor belt or chute which passes along the passageway. Atleast one neutron source and at least two gamma-ray detectors aredisposed within the enclosure to measure gamma-rays produced in the bulkmaterial by both the NIS and TNC processes.

When one neutron source and a gamma-ray detector are arranged in atransmission or backscatter geometry, the spatial response of the gaugewill be biased towards either the source side, the centre or thedetector side of the sample, depending on the relative attenuation ofneutrons and gamma-rays in the sample. The present invention provides anumber of alternative methods to overcome this bias using a seconddetector or second source/detector configuration with a spatial responsebiased to compensate for the spatial bias of the first configuration. Ifthe two configurations used are both transmission or both backscatterthen two sources are required and the source (and detector) of the firstconfiguration will be on the opposite side of the material to the source(and detector) of the second configuration. Variations on the aboveconfigurations involve multiple sources and/or detectors which arelocated across the passageway (perpendicular to the direction of travel)to improve spatial uniformity across the passageway.

Use of the invention can result in significant improvement in spatialuniformity, as a result of the use of two or more of the two possiblesource-sample-detector configurations, viz., backscatter or transmissionconfigurations.

The neutron sources have sufficient energy to excite NIS gamma-rays fromthe element of interest; so called “fast neutron sources”. Neutrons fromthese sources are also slowed to thermal energies in the sample andsurroundings to produce TNC gamma-rays. Suitable sources includeradioisotope sources and neutron generators of either continuous orpulsed mode. The neutron-induced gamma-ray measurements may be combinedwith separate measurements of gamma-ray transmission, thermal neutronflux or fast neutron flux, or any combination of them. NIS measurementsmay be combined with TNC measurements. Suitable high-energy neutronsources are 241Am—Be, a neutron generator or 252Cf, and suitabledetectors are scintillation or solid state detectors such as thalliumactivated sodium iodide NaI(Tl), bismuth germanate BGO or hyperpuregermanium.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to theaccompanying drawings:

FIG. 1 is a schematic drawing of two transmission gauges.

FIG. 2 is a schematic drawing of transmission and backscatter gaugesusing a single fast neutron source.

FIG. 3 is a schematic drawing of two backscatter gauges.

FIG. 4 is a schematic drawing of two transmission and backscattergauges.

FIG. 5a is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 4.438 MeV (C) NIS gamma-rays for a20 cm thick cement raw meal sample, 241Am—Be source(s) and single(dashed) and dual (solid) transmission gauges.

FIG. 5b is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 0.847 MeV (Fe) NIS gamma-rays fora 20 cm thick cement raw meal sample, 241Am—Be source(s) and single(dashed) and dual (solid) transmission gauges.

FIG. 5c is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 2.223 MeV (H) TNC gamma-rays for a20 cm thick cement raw meal sample, 241Am—Be source(s) and single(dashed) and dual (solid) transmission gauges.

FIG. 5d is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 7.645 MeV (Fe) NIS gamma-rays fora 20 cm thick cement raw meal sample, 241Am—Be source(s) and single(dashed) and dual (solid) transmission gauges.

FIG. 6a is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 4.438 MeV (C) NIS gamma-rays for a20 cm thick coal sample using 241Am—Be source(s). The dashed line showsthe results for a single transmission configuration. The solid lineshows the results for dual transmission.

FIG. 6b is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 0.847 MeV (Fe) NIS gamma-rays fora 20 cm thick coal sample using 241Am—Be source(s) The dashed line showsthe results for a single transmission configuration. The solid lineshows the results for dual transmission.

FIG. 6c is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 2.223 MeV (H) TNC gamma-rays for a20 cm thick coal sample using 241Am—Be source(s). The dashed line showsthe results for a single transmission configuration. The solid lineshows the results for dual back-scatter gauges.

FIG. 6d is a graph showing the results of Monte Carlo calculations ofthe relative spatial contributions of 7.645 MeV (Fe) TNC gamma-rays fora 20 cm thick coal sample using 241Am—Be source(s). The dashed lineshows the results for a single transmission configuration. The solidline shows the results for dual back-scatter gauges.

FIG. 7 is a schematic drawing of a practical arrangement of a dualtransmission gauge for on-belt sample measurement of cement raw mealsamples.

FIG. 8 is a schematic drawing of a practical arrangement of dualtransmission/backscatter gauges suitable for the on-line analysis ofcoal.

BEST MODES FOR CARRYING OUT THE INVENTION Achievement of SpatialInsensitivity

An important property of a bulk-material analyser is that the measuredcomposition should not depend on the spatial distribution of differentelements within the sample. This is referred to as spatialinsensitivity. The determination of the elemental composition of asample relies on measuring the number of gamma-rays of characteristicenergy reaching a suitable detector. This means that for a gaugeexhibiting good spatial insensitivity, the probability of a gamma-raybeing produced in a neutron interaction and surviving to reach asuitable detector should be constant within the sample volume.

The rate of production and subsequent detection of gamma-rays depends onthe effective attenuation lengths of neutrons and gamma-rays in thesample and any surrounding material as well as the position of thesources and detectors with respect to the sample and each other. In asingle source-single detector configuration, depending on the relativeeffective attenuation lengths of neutrons and gamma-rays in the sample,material nearest either the source side, the detector side or the centreof the sample will contribute excessively to the observed number ofgamma-rays, thus biasing the composition measurement. Multipletransmission and/or backscattering source/detector configurations can beused to reduce or eliminate this bias.

Examples of such multiple transmission and/or backscatteringconfigurations are as follows:

One neutron source 1 is placed beneath the sample 2 opposite one or moregamma-ray detectors 3 placed above the sample and one neutron source 4placed above the sample 2 opposite one or more gamma-ray detectors 5placed beneath the sample, as shown in FIG. 1. The distance between thetwo source/detector transmission pairs can be selected to optimise thespatial response of the system by balancing contributions from NIS andTNC gamma-rays.

One neutron source 6 is placed beneath the sample 2 opposite one or moregamma-ray detectors 7 placed above the sample and one or more gamma-raydetectors 8 placed beneath the sample shielded 30 from direct radiationfrom the neutron source 6, as shown in FIG. 2.

One neutron source 9 placed beneath the sample 2 adjacent to andshielded 31 from one or more gamma-ray detectors 10 placed beneath thesample and a second neutron source 11 placed above the sample 2 adjacentto and shielded 32 from one or more gamma-ray detectors 12 placed abovethe sample 2, as shown in FIG. 3.

One neutron source 13 placed above the sample 2 opposite a gamma-raydetector 14 placed beneath the sample and a second detector 15 placedabove the sample adjacent to and shielded 33 from the source 13 and asecond neutron source 16 placed beneath the sample opposite a gamma-raydetector 17 placed above the sample 2 and a second detector 18 placedbeneath the sample 2 adjacent to and shielded 34 from the source 16, asshown in FIG. 4.

Demonstration of Method by Monte Carlo Simulation

A deterministic calculation of the rate of gamma-ray production within asample is not in general possible, due to the complexity of theprocesses occurring. Instead, a stochastic (Monte Carlo) samplingprocess can be used to estimate the number of specific gamma-raysproduced per source neutron on a regular array of points within thesample. Given the distance of these grid points from the detector andthe attenuation length of gamma-rays within the sample, the probabilitythat a gamma-ray reaches the detector without interacting can beevaluated deterministically. Combining these results allows the relativeimportance of different regions of the sample to the compositionmeasurement to be calculated.

As an example, a simple geometry, including a rectangular slab of samplematerial, a supporting conveyor belt and a neutron source withappropriate shielding material, has been modelled in a Monte Carloprogram. The rate of production of two typical NIS gamma-rays (0.847 MeViron and 4.438 MeV carbon) and two typical TNC gamma-rays (2.223 MeVhydrogen and 7.645 MeV iron) were determined. These were then multipliedby appropriate gamma-ray attenuation factors to simulate a detectorplaced in a transmission or backscatter configuration. Multiplesource/detector configurations were then constructed from linearcombinations of the single source/detector configurations.

FIGS. 5 and 6 show the result of these studies for two typicalmaterials; cement raw meal, which has a low hydrogen content and hence along neutron attenuation length, and coal which has a higher hydrogencontent. In the figures, the relative contribution of the specifiedgamma-ray has been plotted as a function of depth within the sample. Thedashed lines show the results for a single source/single detectortransmission configuration, and the solid lines the results for a dualarrangement (either dual transmission or dual backscatter). In allcases, the four possible configuration illustrated in FIGS. 1-4 wereevaluated and the one showing the best spatial insensitivity selected.The bias for the single transmission gauge is due to the relativeattenuation of neutrons and gamma-rays together with geometricalfactors. It can be seen that the dual configurations show considerablyimproved spatial insensitivity.

Validation of the Monte Carlo Predictions

In order to validate the predictions of the Monte Carlo models, a seriesof experiments was performed, in which an iron plate was buried atvarious depths inside a silica sample and the iron gamma-raycontribution measured. The calculated and measured gamma-raycontributions from the iron plate are compared in Table 1. Theuncertainties on the results quoted in the ‘Experiment’ column areapproximately 15%. Within this level of error, the Monte Carlo andexperimental data are in good agreement.

TABLE 1 Comparison of Monte Carlo simulation and experimentalmeasurement results for gamma-ray production in an iron plate buriedinside a silica sample. All columns have been scaled to unity for the‘bottom’ position. Number of Relative Position of γ-rays probability ofDetected γ-rays Detected γ- plate inside produced γ-ray reaching (MonteCarlo rays sample in plate detector calculation) (Experiment) Bottom1.00 1.0 1.0 1.0 Middle 0.52 3.0 1.6 1.6 Top 0.20 13.5  2.7 3.2

Determination of Optimum Source/Detector Configuration

For the two sample materials studied, the dual transmissionconfiguration yields the optimum spatial insensitivity for NISgamma-rays. For TNC gamma-rays, the dual transmission configurationperforms best for cement, and the dual backscatter configuration bestfor coal. The preferred geometry for many on-belt cement applicationsinvolving both NIS and TNC gamma-rays is therefore the dual transmissionconfiguration. The preferred geometry for many on-belt coal applicationsinvolving both NIS and TNC gamma-rays is a combination of theconfigurations shown in FIGS. 1 and 3 where the distance between thesources and detectors on the same side of the belt is chosen to achieveoptimum spatial uniformity by balancing contributions from NIS and TNCgamma-rays.

It is envisaged that for measurement of specific elements within samplesof differing composition and thickness, any of the four source/detectorconfigurations illustrated in FIGS. 1-4, or variations thereon asdiscussed above, may prove optimal. Monte Carlo modeling and supportingexperiments can be used to select the best arrangement.

INDUSTRIAL APPLICATIONS

Examples of potential applications are as follows:

The coal industry has an expanding need for on-conveyor belt analysis,particularly for improved product quality control. The requirement isfor better analytical accuracy and better availability of on-linegauges. The parameters of most interest to the coal industry are ash,moisture, sulphur and specific energy. Coal consists of combustible coalmatter (carbon, hydrogen, oxygen and nitrogen) and mineral matter; ashis the oxidised incombustible residue from the combustion of coal

On-line determination of key elements such as iron, silica, alumina,manganese and phosphorus is a key requirement for the improved controlof mining, blending and beneficiation in the iron ore industry. Analysisis generally required in real time directly on high tonnage conveyorbelts.

On-line quality measurement of cement raw materials is a key requirementfor the improved control of cement plants. The primary application to beaddressed is the on-conveyor belt analysis of raw meal to control theraw mix composition. Elements of primary importance in this applicationare calcium, silicon, aluminium and iron.

Previous laboratory and plant work has shown that NIS and TNC techniquesare capable of accurately determining the concentrations of a wide rangeof elements in coal, iron ore and cement raw meal provided the samplesbeing measured are homogeneous.

FIG. 7 illustrates a practical arrangement for installation on aconveyor belt using the same geometry as FIG. 1. The arrangement shownin FIG. 7 is the preferred geometry for many on-belt cement applicationsinvolving both NIS and TNC gamma-rays. Neutron source 20 and gamma-raydetector 21 are housed in separate compartments of a lower shieldedenclosure 22. Gamma-ray detector 23 and neutron source 24 are housed inseparate compartments of an upper shielded enclosure 25. A passageway 26passes between the two shielded enclosures 22 and 25, and a conveyorbelt 27 passes through passageway 26 to carry the sample material 2between the sources and their respective detectors.

FIG. 8 illustrates a practical arrangement for installation on aconveyor belt using the same geometry as FIG. 4 which is suitable foron-belt coal analysis. In this example additional gamma-ray detectors 28and 29 are included in the compartments with neutron sources 20 and 24respectively. Gamma-ray detector 28 is shielded 36 from neutron source20, and gamma-ray detector 29 is shielded 35 from neutron source 24. Theadditional gamma-ray detectors are arranged to measure backscatteredradiation.

It should be understood that the lower and upper shielded enclosures aredesigned to prevent the escape of undesirable radiation, and not toreduce neutron energies before the neutrons enter the measurementvolume.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. For instance, variations where twosources are used and each has an associated transmission and backscatterdetector. The present embodiments are, therefore, to be considered inall respects as illustrative and not restrictive.

What is claimed is:
 1. A bulk material analyser, including: a shieldedenclosure defining an analysis zone within it and having a passagewaythrough it to allow transport of bulk material through the analysiszone; at least one neutron source and at least two gamma-ray detectorsare disposed within the enclosure to measure gamma-rays produced in thebulk material by both the neutron inelastic scatter and thermal neutroncapture processes; a neutron source and a gamma-ray detector arearranged in either a transmission or backscatter geometry; a secondgamma-ray detector is arranged either in a transmission or backscattergeometry with or without using a second neutron source; the arrangementof the detectors being such that the bias in the spatial response ofeach, that is caused by the relative attenuation of neutrons andgamma-rays in the bulk material, at least partly compensates for thebias in the spatial response of the other.
 2. A bulk material analyseraccording to claim 1, wherein the shielded enclosure is made of amaterial which contains a high hydrogen density.
 3. A bulk materialanalyser according to claim 2, wherein the shielded enclosure is made ofa material which contains a high hydrogen density in combination with amaterial of high neutron capture cross section.
 4. A bulk materialanalyser according to claim 3, wherein the material of high neutroncapture cross section is boron or lithium.
 5. A bulk material analyseraccording to claim 1, wherein, in use, the bulk material is transportedthough the analysis zone on a conveyor belt or chute that passes alongthe passageway.
 6. A bulk material analyser according to claim 1,wherein one neutron source is placed beneath the passageway opposite agamma-ray detector placed above the passageway and one neutron source isplaced above the passageway opposite a gamma-ray detector placed beneaththe passageway.
 7. A bulk material analyser according to claim 1,wherein one neutron source is placed beneath the passageway opposite agamma-ray detector placed above the passageway and one gamma-raydetector is placed beneath the passageway shielded from direct radiationfrom the neutron source.
 8. A bulk material analyser according to claim1, wherein one neutron source is placed above the passageway adjacent toand shielded from a gamma-ray detector, and another neutron source isplaced below the passageway adjacent to and shielded from anothergamma-ray detector.
 9. A bulk material analyser according to claim 1,wherein one neutron source is placed above the passageway adjacent toand shielded from a first gamma-ray detector, and a second gamma-raydetector is placed beneath the passageway; and a second neutron sourceis placed below the passageway adjacent to an shielded from a thirdgamma-ray detector, and a fourth gamma-ray detector is placed above thepassageway.
 10. A bulk material analyser according to claim 1, whereinone neutron source is placed above the passageway opposite a gamma-raydetector placed beneath the passageway and a second neutron source isplaced beneath the passageway adjacent to and shielded from gamma-raydetector placed beneath the passageway.
 11. A bulk material analyseraccording to claim 1 where the distance between the sources anddetectors on the same side of the belt is chosen to achieve optimumspatial uniformity.
 12. A bulk material analyser according to claim 1wherein the neutron sources have sufficient energy to excite NISgamma-rays from the element of interest.
 13. A bulk material analyseraccording to claim 1, wherein hydrogenous material is provided aroundthe neutron source to reduce the average neutron energy from the source.14. A bulk material analyser according to claim 1, wherein the neutronsource is a radioisotope source, or an RF linac, or a neutron generatorin either continuous or pulsed mode.
 15. A bulk material analyseraccording to claim 1, wherein the neutron-induced gamma-ray measurementsare combined with separate measurements of gamma-ray transmission,thermal neutron flux or fast neutron flux, or any combination of them.16. A bulk material analyser according to claim 1, wherein pulsescomprising the output from the detectors are amplified by means of asuitable amplifier, and resultant amplified spectra then are processedusing a suitable computer to produce an output indicative of theelemental composition of the material passing through the analyser. 17.A bulk material analyser according to claim 1, wherein the analyserutilises a high-energy neutron source such as 241Am—Be, a neutrongenerator or 252 Cf and scintillation or solid state detectors such asthallium activated sodium iodide (NaI(Tl)), bismuth germanate (BGO) orhyperpure germanium.
 18. A bulk material analyser according to claim 1,wherein the NIS measurements are combined with TNC measurements.
 19. Abulk material analyser according to claim 1, wherein additional sourcesare positioned laterally across the passageway on the same side of thepassageway as the existing sources.
 20. A bulk material analyseraccording to claim 1, wherein additional gamma-ray detectors arepositioned laterally across the passageway on the same side of thepassageway as the existing detectors.
 21. A bulk material analyseraccording to claim 1, where uniformity of spatial response of TNCgamma-rays is improved by the use of neutron moderators and reflectorsplaced around the source and sample.